Well construction has a number of well-known problems that can affect the ability to recover oil from the formation through which the well is drilled, or even, in extreme, circumstances to complete the well and bring it to production.
When drilling horizontal wells in oil-bearing formations, a key factor for success is to try to keep the well a constant distance above the water table that underlies the oil. When this is not achieved and the well trajectory varies, the low points or ‘valleys’ of the well are often sources of problem. If open-hole completion is used or if the perforation density in that region is uniform, there is a high risk of water coning towards the well valley. Even with cased hole and no perforation in the valleys, some lengths of well can still lose contact with the reservoir. In rare situations, a drilled section of well must be abandoned and side-tracking is performed to re-position the well at the correct depth.
A similar problem may appear in the “up-hill” part of the well when the distance from the well to gas layer overlying the oil is too small. In this case gas can then be produced, with similar consequence and treatment as the “valley” and water problem described above.
In some wells, the drilling process itself generates some formation damage in the near well-bore region. This appears as a high skin effect with a consequential production limitation. Certain chemical treatments have previously been proposed to be performed in the rock matrix for cleaning the rock pores and re-establishing the proper permeability but these are not always effective.
For sand control during production, the common solution is to use gravel packing and screens. In horizontal wells, the placement of the gravel can be quite challenging, while at the same time reducing the flow section (the open bore of the well) in the completion. For completions based on gravel packing and fracturing (“Pack&Frac” technique), there can be difficulties with the placement of the pack, and there is no control of the direction of the short fractures produced.
For the problem of formation collapse due to stresses in the rock (and stress concentration near the well bore), the only solutions are either to adapt mud density used during the drilling of the well with risk of fracturing other layers, or to abandon this section of the well and restart with another well bore trajectory.
For loss of drilling fluid while drilling, the problem is often solved by placing some cement slurry at the bottom of well and squeezing part of it in the formation. However, the resulting treatment is often not very deep and when the drilling is reinitiated across the cement plug, the well bore can enter virgin formation again, and losses often restart.
The problem of narrow pressure window is often difficult to solve: there is limited freedom to adjust mud density while avoiding formation fracturing or influx of formation fluid in the well-bore. Often a casing has to be installed to isolate that formation.
It is an object of the invention to provide constructions techniques that are alternatives to these treatments of methods and which can potentially overcome some or all of the problems. The invention is based on the use of lateral boreholes, i.e. secondary boreholes that are drilled from a main borehole. Laterals have been previously proposed for various uses, in particular for providing improved contact with the formation.